FIG. 1 Dallas Mill Creek drainage tunnel alignment. The Dallas Mill Creek Drainage Relief Tunnel is an 8.4 km (5 mile) tunnel that will provide 100-year flood protection for impacted neighborhoods in the east Dallas area. \u201cBig Tex\u201d is a main beam gripper tunnel boring machine (TBM) capable of changing diameters in mid-tunnel to excavate 2.83 km (9,290 ft) of 11.45 m (37.58 ft) diameter tunnel and 5.21 km (17,095 LF) of 9.93 m (32.58 ft) diameter tunnel. Construction began in April 2018 and is anticipated to be completed in 2024. This article will discuss the challenges and lessons learned during assembly, reassembly, startup and mining with Big Tex. History and background The city of Dallas has a long history of severe flooding over the years that has resulted in property damage, injuries and even deaths. Efforts to mitigate the flooding included a series of levees built by the U.S. Army Corps of Engineers in the early 1900s along the Trinity River that evolved into what is today the Dallas floodway. A 1995 flooding event, caused by a series of heavy rainstorms throughout the Dallas metro area, overwhelmed the storm drains and box culverts installed in the 1930s and 1940s and flooded streets in some areas with up to 3 m (10 feet) of water. This devastating event resulted in the deaths of 15 people, injuries to more than 100 people, and severe damage to hundreds of homes and businesses, including flooding the emergency room of a major hospital in Dallas, Baylor University Medical Center. Subsequent severe storm events in 2006 and 2007 accelerated the level of planning and design for flood relief using stormwater diversion and conveyance to the Trinity River. The city looked at the drainage areas most impacted by severe flooding and selected the Mill Creek, Peaks Branch, East Peaks Branch, and State Thomas drainage areas for stormwater flood mitigation. The city and design engineer assigned to the project collaborated to develop and evaluate separate conceptual designs for three of the four drainage areas, but construction costs, environmental impacts, the inconvenience of expected large cut and cover trench construction, various detours and multiple road closures resulted in combining the three separate drainage designs and the State Thomas drainage area into a deep drainage tunnel discharging into White Rock Creek. After completion of a detailed boring program, the final design resulted in the 8.4 km (5 miles) long Dallas Mill Creek Drainage Relief Tunnel as shown (Fig. 1). The project After more than 10 years of planning and design, the Dallas Mill Creek Drainage Relief Tunnel project was finally let out for bid in September 2015 followed by bid openings in December 2015. However, the decision was made to reject all bids due to bid irregularities, revise the plans, revise the bidding process and rebid the project. The city also replaced the construction management firm. The city, design engineer and new construction management firm revised the bid documents and advertised the project in July 2017. The city successfully awarded the Dallas Mill Creek Drainage Relief Tunnel Project to Southland Mole JV (SMJV) in February 2018 and awarded notice to proceed in March 2018. FIG. 2 Outfall shaft site. FIG. 3 Robbins 11.5 m TBM \u201cBig Tex\u201d assembled on the surface. Outfall shaft site The outfall shaft is the design discharge point of the Dallas Mill Creek Drainage Tunnel and will be the main working shaft during construction of the tunnel. The outfall shaft site is located at the end of Barber Avenue on property purchased by the City of Dallas (Fig. 2). Large diameter tunnel and shaft projects typically require large work sites because of the lay down areas required for specialty equipment and to enable room for operations facilities as well as tunnel muck stockpiles and muck handling areas. To secure the land required for the project, the city purchased as many properties located along Barber Avenue as possible, but not all residents accepted purchase offers. In anticipation of multiple truck loads 12-hours a day on Barber Avenue, the city and design engineer added Barber Avenue street improvements, including utilities and new pavement to the contract. Challenging muck limitations and handling SMJV realized the limited size and location of the outfall shaft site would impact TBM production because of muck storage limitations and muck handling and hauling restrictions from 7 a.m. to 7 p.m. due to noise ordinances on city streets. TABLE 1 Tunnel muck handling, 24 hour and 12 hour. SMJV proposed changing the location of the muck shaft operation from Site O to a location across Scyene Road (state route 352) approximately 594 m (1,950 ft) up station from the outfall shaft. SMJV negotiated a land use agreement with the property owner of the muck shaft site that resulted in a win-win situation for the city\u2019s residential neighborhood along Barber Avenue and Dixon Avenue, and for SMJV, as it would allow SMJV to mine and haul tunnel muck 24-hours per day. FIG.4 TBM trailing gantry assembly on rail and slab. The proposed change would eliminate heavy truck traffic on residential streets and allowed for hauling muck 24-hours a day, six days a week on a state highway (Table 1). The Dallas Mill Creek Drainage Relief Tunnel was designed with two different cross-sections to handle peak flows of 424.7 m3\/sec (15,000 cu ft\/sec) through the upper reach and 566.3 m3\/sec (20,000 cu ft\/sec) through the lower reach. FIG.5 Lowering rear main beam section with grippers and gripper carrier. The upper reach is a 9.1 m (30 ft) finished dimeter circular tunnel 5,212 m (17,095 ft) long and the lower reach is a 9.1 m \u00d7 10.8 m (30 ft \u00d7 35.5 ft) finished horseshoe tunnel 2,750 m (9,019 ft) long. It was thought that the contractor would excavate the complete tunnel with a TBM from the outfall shaft to the Woodall Rodgers intake shaft and then complete the horseshoe tunnel bottom excavation by roadheader or excavator. The horseshoe tunnel liner would require additional concrete forms and multiple diameter placements that would result in increased costs and increased schedule. TBM assembly and TBM launch The Robbins Co. (TRC) and SMJV elected to assemble the TBM onsite using TRC\u2019s patented onsite first-time assembly delivery method. Deliver TBM components and assemble the TBM onsite (Fig. 3).Commission, startup and test various systems. Provide \u201cfactory\u201d test reports.Reassemble the TBM in the starter tunnel, complete final testing and provide final factory test reports. SMJV installed a reinforced concrete slab system to assist TRC with the TBM onsite assembly and trailing gear on. The 1,133 t (1,115 st) TBM (cutterhead, cutterhead support, main bearing, mainframe, finger shields, grippers, gripper carrier, gripper feet, etc.) was assembled on a 0.61 m (2 ft) thick 31 MPa (4,500 psi) concrete slab with a double mat of reinforcement. The four trailing gantries were fully assembled on a 0.3 m (1 ft) thick concrete slab with a single mat of reinforcement and rolled onto an 85 lb. rail that simulated the 2.9 m (9.5 ft) gauge leapfrog rail system for the gantries to roll on during excavation. (Fig. 4). The various components and pieces of the TBM, including all of the parts for the four gantries, the vertical conveyor system and the continuous horizontal conveyor system began arriving onsite in February 2019. FIG.6 Starter tunnel with curved pocket and gripper walls. Delayed delivery of major TBM parts, including the mainframe, grippers, main bearing and cutterhead support, drive motors and cutterhead resulted in significant slippage of the project schedule and resequencing of the TBM assembly by TRC and SMJV. To accelerate the disassembly and reassembly in the starter tunnel, TRC and SMJV used a 653-t (720-st) Manitowoc MLC650 super-lift crane to make eight heavy-lift picks of the TBM and gantries (Fig. 5). The four TBM heavy picks included the cutterhead at 190.5 t (187.5 st), the cutterhead support with main bearing at 389 t (375 st), the front main beam section at 127 t (125 st) and the rear main beam section with grippers and gripper carrier at 228.6 t (225 st). The four gantry heavy lifts included each gantry assembled and fully dressed weighing in at an average of 91 t (90 st). Prior to lifting the gantries, all utilities including electric, hydraulics, communications, ventilation ducts, etc. were disconnected. All eight heavy lifts were sequentially lowered down the excavated 14 m (46 ft) diameter outfall shaft and assembled in the 76.2 m (250 ft) long \u00d7 12.2 m (40 ft) horseshoe starter tunnel. FIG.7 Last gantry in starter tunnel on leapfrog rail. The reassembled TBM was moved to the face of the starter tunnel on a curved invert cast in the working slab (Fig. 6) followed by rolling the four gantries on the leapfrog rail system (Fig. 7). The TBM was launched from STA 10+71\u00b1 after final testing was complete, and the TBM was certified as ready to go and the muck conveyance system was installed. The muck conveyance system is comprised of seven belt conveyors, an advancing tail piece, and a double-stacked horizontal belt storage unit on its surface. TBM conveyor. Transfer crossover conveyor. Gantry transfer conveyor. Horizontal continuous conveyor. Vertical conveyor. Surface transfer conveyor. Radial stacker conveyor. SMJV, with TRC on board, advanced the TBM for the first 305 m (1,000 ft) to confirm and verify the capabilities of the TBM per the contract. SMJV continued advancing the TBM and was scheduled to relocate the mucking operation (vertical conveyor and radial stacker conveyor) from the outfall shaft to the muck shaft after reaching STA 30+50\u00b1. In an effort to mitigate project schedule slippage, SMJV and TRC elected to relocate the mucking operation concurrently during the scheduled TBM cutterhead diameter conversion in late December 2020, however SMJV and TRC reversed their decision and kept the mucking operation at the outfall shaft. The tunnel is being excavated through Austin Chalk, a geological formation in the Gulf Coast region of the United States made up of weak limestone with an average unconfined compressive strength of 24.8 MPa (3,597 psi) at depths of 33.5 m to 51.7 m (110 ft to 170 ft) below the surface (Fig. 8). FIG.8 Dallas Mill Creek drainage tunnel geotechnical profile. SMJV is using an average TBM production rate of 24.4 m\/day (80 ft\/day) for the project schedule that is based on previous tunnel projects in the Dallas area that occurred within the same geology. however SMJV has not been able to achive that production rate. The excavation will generate 1.15 million m\u00b3 (1.5 million cy) of tunnel muck. In an effort to maintain the scheduled advance rate of 24.4 m\/day (80 ft\/day) and not get muck bound, an average of 230 trailer dump loads per day of muck must be removed from the outfall shaft site. FIG.9 Plan and section of cutterhead diameter conversion at lateral A. FIG.10 Section profile of cutterhead diameter conversion at lateral A. SMJV has experienced up to 400 trailer dump loads per day. The relocated muck shaft site would have enough surface area available to stockpile up to three days of excavated tunnel muck. TBM cutterhead diameter conversion in midtunnel To mitigate the extra work, time and cost of the lower reach 9.1 m \u00d7 10.8 m (30 ft \u00d7 35.5 ft) finished horseshoe tunnel, SMJV proposed to change the 2,749 m (9,017 ft) lower reach from the horseshoe tunnel to a 10.7 m (35 ft) finished diameter circular tunnel by using a Robbins hard-rock gripper TBM designed to reduce cutterhead diameters from 11.5 m to 9.9 m (37.6 ft to 32.6 ft) in the tunnel underground at STA 101+55\u00b1, the beginning of the lower reach. The larger diameter cutterhead uses 67 432 mm (17 in.) disk cutters and the reduced diameter cutterhead will use 59 432 mm (17 in.) disk cutters. The end of the lower reach location includes the East Peaks branch intake structure, shaft A, lateral A, lateral P and shaft P. SMJV and TRC set out to complete the TBM diameter conversion from the larger diameter to the smaller diameter through a sequence developed by SMJV and TRC that included mining to a specific station and sequentially removing (unbolting) the 0.76 m (2.5 ft) cutterhead extensions and the outer cutterhead quadrants while backing the TBM up approximately 31.4 m\u00b1 (103 ft) to the lateral A transition area at STA 100+52\u00b1. SMJV continued the diameter conversion by cutting (plasma torching) the 0.76 m (2.5 ft) extension outer wraps off the grippers, cutterhead support sidestabilizers, the three roof finger shields and the rear support (gripper feet). FIG.11 Cutterhead diameter conversion area. The 0.76 m (2.5 ft) cutterhead extensions, outer cutterhead quadrants and cut pieces were removed through the 3.7 m (12 ft) horseshoe tunnel extension of lateral A and excavated after the right gripper passed lateral A (Figs. 9 and 10). The cutterhead extensions, outer cutterhead quadrants and cut pieces were removed through the 91 m (300 ft) long 6.1 m (20 ft) diameter lateral A and out the 6.1 m (20 ft) diameter shaft A. The cutterhead quadrant buckets required additional machine work to meet the smaller diameter. The TBM was moved forward as the outer cutterhead quadrants were rebolted sequentially to the center section and the leapfrog rail and trailing gantries blocked up to line up with the new diameter. The gripper outer wraps and cutterhead support side stabilizer outer wraps were cut and pinned to the tunnel wall in the 11.5 m (37.6 ft) tunnel prior to the main beam with grippers sliding by. FIG.12 Dallas Mill Creek Drainage Tunnel mega hard rock TBM \u201cBig Tex\u201d SMJV resumed mining with the 9.9 m (32.6 ft) diameter TBM on a 1 percent grade to STA 105+00\u00b1 where the TBM was back on contract grade (Fig. 11). Shafts, intake structures and dewatering pump station The drainage tunnel, with a capacity of 605,666 m\u00b3 (160 million gal), has six shafts that vary from 3.8 m to 12.2 m (12.5 ft to 40 ft) in diameter and are 33.5 m to 57 m (110 ft to 187 ft) in depth. There are five intake structures that intersect five existing storm box culverts and will convey stormwater to the tunnel via vortex drop shafts and laterals. The horseshoe laterals vary from 15.8 m to 149.4 m (52 ft to 490 ft) in length and 3.66 m to 6.1 m (12 ft to 20 ft) in height. There is a dewatering pump station in shaft P at the East Peaks branch intake designed to dewater the tunnel for maintenance and inspection. The pump station has three submersible pumps with a design capacity of 113.6 m\u00b3\/min (30,000 gal\/min) located in shaft P, the deepest part of the drainage tunnel at 57 m (187 ft) deep. The pump station is designed to dewater the inverted siphon conveyance tunnel within three to five days during dry periods. Conclusion The Dallas Mill Creek Drainage Tunnel will be the largest hard rock gripper TBM tunnel under construction in the Western hemisphere, and possibly the world. There were valuable lessons learned on this project by everyone involved during site mobilization, shaft construction, large diameter TBM assembly, launching the TBM, excavating the Dallas Mill Creek Drainage Tunnel with \u201cBig Tex\u201d (Fig. 12), and preparing for the unique cutterhead diameter conversion in mid-tunnel. The expected and unexpected challenges encountered during the start-up and production mining of the Dallas Mill Creek Drainage Tunnel were solved by collaboration and communication between the experienced individuals involved with the Dallas Mill Creek Drainage Tunnel team. The proposed relocation of the muck shaft to accommodate hauling 24-hours per day on a state highway, eliminating the large horseshoe tunnel construction with a round tunnel construction, and the innovative changing TBM cutterhead diameters in mid-tunnel should result in a more efficient excavation operation and should save time on the construction schedule. The city, construction management team and SMJV were able to work through issues while maintaining an open line of communication that helped keep the project moving forward during difficult times. Many thanks to everyone directly and indirectly involved in this challenging world class project.